Isaiah R. Turnbull

The TREM receptor family and signal integration.

TREM proteins are a family of cell surface receptors that participate in diverse cell processes, including inflammation, bone homeostasis, neurological development and coagulation. TREM-1, the first to be identified, acts to amplify inflammation. Other TREM proteins regulate the differentiation and function of macrophages, microglia, dendritic cells, osteoclasts and platelets. Here we discuss the state of the field, putative ligands of TREM proteins and the challenges that remain in understanding TREM biology.

Cutting Edge: TREM-2 Attenuates Macrophage Activation

The triggering receptor expressed on myeloid cells 2 (TREM-2) delivers intracellular signals through the adaptor DAP12 to regulate myeloid cell function both within and outside the immune system. The role of TREM-2 in immunity has been obscured by the failure to detect expression of the TREM-2 protein in vivo. In this study, we show that TREM-2 is expressed on macrophages infiltrating the tissues from the circulation and that alternative activation with IL-4 can induce TREM-2. TREM-2 expression is abrogated by macrophage maturation with LPS of IFN-γ. Using TREM-2−/− mice, we find that TREM-2 functions to inhibit cytokine production by macrophages in response to the TLR ligands LPS, zymosan, and CpG. Furthermore, we find that TREM-2 completely accounts for the increased cytokine production previously reported by DAP12−/− macrophages. Taken together, these data show that TREM-2 is expressed on newly differentiated and alternatively activated macrophages and functions to restrain macrophage activation.

Activating and inhibitory functions of DAP12

When associated with different receptors, the signalling adaptor DAP12 has been shown to both potentiate and attenuate the activation of leukocytes. But how can a protein with a single signalling motif elicit qualitatively different cellular responses? We describe a model of DAP12 function, whereby the quality of the cellular response (activation or inhibition) is modulated by the avidity of the interaction between the DAP12-associated receptor and its ligand. This model extends from previous studies of inhibitory signalling mediated by other adaptors, such as the Fc-receptor γ-chain and CD3ζ, and provides a potential mechanism for the conflicting phenotypes observed in studies of DAP12-deficient mice.

Macrophage colony-stimulating factor induces the proliferation and survival of macrophages via a pathway involving DAP12 and |[beta]|-catenin

Macrophage colony-stimulating factor (M-CSF) influences the proliferation and survival of mononuclear phagocytes through the receptor CSF-1R. The adaptor protein DAP12 is critical for the function of mononuclear phagocytes. DAP12-mutant mice and humans have defects in osteoclasts and microglia, as well as brain and bone abnormalities. Here we show DAP12 deficiency impaired the M-CSF-induced proliferation and survival of macrophages in vitro. DAP12-deficient mice had fewer microglia in defined central nervous system areas, and DAP12-deficient progenitors regenerated myeloid cells inefficiently after bone marrow transplantation. Signaling by M-CSF through CSF-1R induced the stabilization and nuclear translocation of β-catenin, which activated genes involved in the cell cycle. DAP12 was essential for phosphorylation and nuclear accumulation of β-catenin. Our results provide a mechanistic explanation for the many defects of DAP12-deficient mononuclear phagocytes.

Sepsis and septic shock

For more than two decades, sepsis was defined as a microbial infection that produces fever (or hypothermia), tachycardia, tachypnoea and blood leukocyte changes. Sepsis is now increasingly being considered a dysregulated systemic inflammatory and immune response to microbial invasion that produces organ injury for which mortality rates are declining to 15–25%. Septic shock remains defined as sepsis with hyperlactataemia and concurrent hypotension requiring vasopressor therapy, with in-hospital mortality rates approaching 30–50%. With earlier recognition and more compliance to best practices, sepsis has become less of an immediate life-threatening disorder and more of a long-term chronic critical illness, often associated with prolonged inflammation, immune suppression, organ injury and lean tissue wasting. Furthermore, patients who survive sepsis have continuing risk of mortality after discharge, as well as long-term cognitive and functional deficits. Earlier recognition and improved implementation of best practices have reduced in-hospital mortality, but results from the use of immunomodulatory agents to date have been disappointing. Similarly, no biomarker can definitely diagnose sepsis or predict its clinical outcome. Because of its complexity, improvements in sepsis outcomes are likely to continue to be slow and incremental.

Effects of age on mortality and antibiotic efficacy in cecal ligation and puncture.

The incidence and mortality of sepsis increase with age, consequently, 80% of the clinical mortality from sepsis occurs in patients over age 65. Despite this aged clinical population, most research models of sepsis use 6- to 16-week-old mice as patient surrogates. This age range of mice corresponds to human ages 10 to 17 years. To assess the influence of age on rodent CLP and on antibiotic therapy, we studied young (4 month), mature (12 month), and aged (24 month) mice. Male C57BL/6 mice (n = 27–30 in each age group) were subjected to cecal ligation and puncture (CLP), two punctures with a 25-gauge needle. Mice were observed untreated for 10 days. Young mice had 20% mortality, mature mice had 70% mortality (P = 0.0013 vs. young), and aged mice had 75% mortality (P = 0.0001 vs. young). To assess the effects of age on antibiotic therapy, mice were subjected to CLP as above (n = 38–40 in each age group). Mice were then randomized to treatment with intraperitoneal injections of ceftriaxone and metronidazole or normal saline. Therapy was initiated 12 h after CLP, and injections were repeated every 12 h for 7 days. Young mice saw a 56% decrease in mortality from CLP with antibiotic therapy (P = 0.001), and mature mice had a 30% decrease in mortality (P = 0.06). Aged mice saw no benefit from antibiotic therapy. We also compared plasma cytokine levels between young and aged mice after CLP. When compared with young mice, aged mice had higher levels of IL-6 and TNF-&agr; 24 h after CLP. However, high IL-6 was predictive of mortality at any age. Mice appear to have age-dependent responses to intra-abdominal sepsis and to appropriate therapy.

Antibiotics improve survival in sepsis independent of injury severity but do not change mortality in mice with markedly elevated interleukin 6 levels.

Genetically identical mice have a heterogeneous response to antibiotic therapy in sepsis, with only a subset deriving therapeutic benefit. We sought to determine whether the severity of a septic insult correlates with the survival benefit conferred by antibiotics. We also sought to determine whether antibiotics given 12 h after injury alter survival in animals predicted to die based upon high interleukin (IL)-6 levels drawn 6 h earlier. Adult male ND4 mice (n = 363) were subjected to double-puncture cecal ligation and puncture (CLP) with a 19-, 21-, or 23-gauge needle. Animals were randomized to receive imipenem or 0.9% NaCl every 12 h after CLP for 5 days. Ten-day survival was 16%, 26%, and 52%, respectively, for untreated animals. Antibiotics decreased the absolute risk of death 17% to 23% regardless of injury severity. In a separate cohort, mice (n = 37) were subjected to single or double-puncture CLP with a 21-gauge needle. IL-6 levels were assayed 6 h postoperatively and mice were followed for survival. Levels greater than 14,000 pg/mL were identified as predicting a 100% mortality (7/7 animals dead). A third set of mice (n = 94) then underwent double-puncture CLP with either 21-, 23-, or 25-gauge needle and had IL-6 levels measured in a similar fashion. Animals were randomized to receive imipenem or 0.9% NaCl beginning 12 h postoperatively (6 h after IL-6 levels were drawn) and continued for 5 days or until death. Although antibiotics decreased mortality overall, all animals with IL-6 levels greater than 14,000 pg/mL (n = 13) died, regardless of whether they received antibiotics or the gauge of needle used. These results indicate that antibiotics improve outcome in murine sepsis, regardless of injury severity. Furthermore, there is a threshold IL-6 level that can be identified 6 h after sepsis above which animals are destined to die, and antibiotic treatment does not alter their outcome.

DAP12 (KARAP) amplifies inflammation and increases mortality from endotoxemia and septic peritonitis

DAP12 (KARAP) is a transmembrane signaling adaptor for a family of innate immunoreceptors that have been shown to activate granulocytes and monocytes/macrophages, amplifying production of inflammatory cytokines. Contrasting with these data, recent studies suggest that DAP12 signaling has an inhibitory role in the macrophage response to microbial products (Hamerman, J.A., N.K. Tchao, C.A. Lowell, and L.L. Lanier. 2005. Nat. Immunol. 6:579–586). To determine the in vivo role for DAP12 signaling in inflammation, we measured the response of wild-type (WT) and DAP12−/− mice to septic shock. We show that DAP12−/− mice have improved survival from both endotoxemia and cecal ligation and puncture–induced septic shock. As compared with WT mice, DAP12−/− mice have decreased plasma cytokine levels and a decreased acute phase response during sepsis, but no defect in the recruitment of cells or bacterial control. In cells isolated after sepsis and stimulated ex vivo, DAP12 signaling augments lipopolysaccharide-mediated cytokine production. These data demonstrate that, during sepsis, DAP12 signaling augments the response to microbial products, amplifying inflammation and contributing to mortality.

Effects of aging on the immunopathologic response to sepsis.

Objective:Aging is associated with increased inflammation following sepsis. The purpose of this study was to determine whether this represents a fundamental age-based difference in the host response or is secondary to the increased mortality seen in aged hosts. Design:Prospective, randomized controlled study. Setting:Animal laboratory in a university medical center. Subjects:Young (6–12 weeks) and aged (20–24 months) FVB/N mice. Interventions:Mice were subjected to 2 × 25 or 1 × 30 cecal ligation and puncture (CLP). Measurements and Main Results:Survival was similar in young mice subjected to 2 × 25 CLP and aged mice subjected to 1 × 30 CLP (p = 0.15). Young mice subjected to 1 × 30 CLP had improved survival compared with the other groups (p < 0.05). When injury was held constant but mortality was greater, both systemic and peritoneal levels of tumor necrosis factor-α, interleukin (IL)-6, IL-10, and monocyte chemotactic protein-1 were elevated 24 hours after CLP in aged animals compared with young animals (p < 0.05). When mortality was similar but injury severity was different, there were no significant differences in systemic cytokines between aged mice and young mice. In contrast, peritoneal levels of tumor necrosis factor-α, IL-6, and IL-10 were higher in aged mice subjected to 1 × 30 CLP than young mice subjected to 2 × 25 CLP despite their similar mortalities (p < 0.05). There were no significant differences in either bacteremia or peritoneal cultures when animals of different ages sustained similar injuries or had different injuries with similar mortalities. Conclusions:Aged mice are more likely to die of sepsis than young mice when subjected to an equivalent insult, and this is associated with increases in both systemic and local inflammation. There is an exaggerated local but not systemic inflammatory response in aged mice compared with young mice when mortality is similar. This suggests that systemic processes that culminate in death may be age independent, but the local inflammatory response may be greater with aging.

Proteasome activator PA200 is required for normal spermatogenesis.

ABSTRACT The PA200 proteasome activator is a broadly expressed nuclear protein. Although how PA200 normally functions is not fully understood, it has been suggested to be involved in the repair of DNA double-strand breaks (DSBs). The PA200 gene (Psme4) is composed of 45 coding exons spanning 108 kb on mouse chromosome 11. We generated a PA200 null allele (PA200Δ) through Cre-loxP-mediated interchromosomal recombination after targeting loxP sites at either end of the locus. PA200Δ/Δ mice are viable and have no obvious developmental abnormalities. Both lymphocyte development and immunoglobulin class switching, which rely on the generation and repair of DNA DSBs, are unperturbed in PA200Δ/Δ mice. Additionally, PA200Δ/Δ embryonic stem cells do not exhibit increased sensitivity to either ionizing radiation or bleomycin. Thus, PA200 is not essential for the repair of DNA DSBs generated in these settings. Notably, loss of PA200 led to a marked reduction in male, but not female, fertility. This was due to defects in spermatogenesis observed in meiotic spermatocytes and during the maturation of postmeiotic haploid spermatids. Thus, PA200 serves an important nonredundant function during spermatogenesis, suggesting that the efficient generation of male gametes has distinct protein metabolic requirements.